1. Field of the Invention
The present invention relates to a strain detector for measuring an external stress by converting strain generated in a strain generating body to an electric signal, and a method of manufacturing the same.
2. Description of Related Art
There have been developed various types of strain detectors for measuring an external stress using a strain generating body in which a component is strained according to an external stress, and a strain gauge for converting a strain of the strain generating body to an electric signal. For instance, in a strain detector currently available for measuring the pressure of a fluid such as a gas or a liquid, a strain generating body is strained by the pressure of a fluid, and the strain is converted to an electric signal indicating the pressure of the fluid.
As the strain detector for measuring the pressure of a fluid, there have been known, for instance, a strain detector disclosed in the document: Japanese Patent Laid-Open Publication No. 2001-242031. This strain detector includes a diaphragm portion for generating strain according to the pressure of a fluid and a strain gauge for detecting the generated strain as an electric signal, and is used for measuring the pressure of a fluid flowing through piping. The diaphragm portion is made of 17-4 stainless steel, and has a thin wall portion which is elastically deformed by the pressure of a fluid and generates strain. The strain gauge is prepared by adhering a plurality of resistance wire strain gauges on the thin wall portion respectively with a glass material to form a circuit. This glass material functions as an adhesive between the thin wall portion and the resistance wire strain gauge, and at the same time functions as an insulating material for shutting off electric conduction between the thin wall portion and the resistance wire strain gauge. In the strain detector having the configuration as described above, when a fluid, which is the object to be measured, acts to the thin wall portion of the diaphragm portion, the thin wall portion is strained due to the pressure of the fluid, and the resistance wire strain gauge detects the strain as an electric signal, amplifies and converts this electric signal, thus a pressure value of the fluid is obtained.
When it is tried to detect a pressure value for a fluid with higher precision, it is preferable to generate a higher strain, and for that purpose, the thickness of the thin wall portion of the diaphragm portion should preferably be as small as possible. Further, since this thin wall portion receives high pressure from a fluid, which is the object to be measured, the material for the diaphragm portion is required to have a high tensile strength. To satisfy the requirement, 17-4 stainless steel, which is martensitic precipitation hardening type stainless steel, is used as a material for the diaphragm, and because this material has the high tensile strength of 1300 MPa or more, a highly accurate measured value can be obtained, and the 17-4-stainless steel is well suited to be used as a material for the diaphragm of a strain detector.
However, although this martensitic precipitation hardening type stainless steel is excellent in the mechanical strength, the corrosion resistance is not so high, and therefore availability of the strain detector disclosed in the document is limited especially when measuring the pressure of fluids such as highly corrosive acidic gases, chemical agents, or hydrogen gas which causes embrittlement, or the like.
Further, when the thin wall portion with smaller thickness is formed by using a material having the high tensile strength such as the martensitic precipitation hardening type stainless steel in the diaphragm, the thin wall portion can generate large strain, but the glass material used for jointing the thin wall portion and the resistance wire strain gauge receives a large stress from the thin wall portion.
In the conventional technology for strain gauging including an aforesaid strain gauge, the glass material or a polyimide-based adhesive is used for junction between the resistance wire strain gauge and the diaphragm. When a strain detector made by using the aforesaid adhesive in the aforesaid diaphragm portion is used for a long time, juncture between the diaphragm and the resistance wire strain gauge becomes weaker, and strain of the diaphragm can not accurately be detected as an electric signal, namely a highly reliable measured pressure value can no longer be obtained. Further, since the strain detector requires the work for adhering a plurality of resistance strain gauges discretely with an adhesive, manufacture of the strain detector takes a long period of time and much labor load, which are also disadvantageous.
The problems as described above are not limited to the field of strain detectors for measuring the pressure a fluid, and also other general strain detectors have the same problems